14:30 - 14:35 | Session Welcome
Abstract: Welcome to our splinter session.
14:35 - 15:00 | The implications of cool star multiplicity statistics for star formation theories
Cathie Clark (University of Cambridge)
Abstract: It has long been recognised that the assembly of multiplicity statistics
for a large dynamic range of primary masses provides a tool for distinguishing
various star formation scenarios. I will review the extent to which we currently
have the statistical information to conduct this analysis and highlight future
observational needs. I will also discuss the long-standing problem of creating
extreme mass ratio binaries and describe scenarios explored in the recent theoretical
litereature. Finally I will review the extent to which data on circumstellar
discs in young brown dwarfs can constrain the dynamical environment in which they formed.
15:00 - 15:14 | Accretion Dynamics in Pre-Main Sequence Binary Stars: Observationally Testing the Accretion Stream Theory
Ben Tofflemire (University of Wisconsin, Madison)
Abstract: Protostellar disks are integral to the formation of low-mass stars and planets. A paradigm for the star-disk interaction has been extensively developed through theory and observation in the case of single stars. Most stars, however, form in binaries or higher order systems where the distribution of disk material and mass flows are more complex. Pre-main sequence (pre-MS) binary stars can have up to three accretion disks: two circumstellar disks and a circumbinary disk separated by a dynamically cleared gap. Theory suggests that orbital motion may drive periodic accretion streams that flow from a circumbinary disk across the gap onto the circumstellar disks or stellar surfaces. Thus, accretion in pre-MS binaries is controlled not only by radiation, disk viscosity, and magnetic fields, but also by orbital dynamics. As part of a larger, ongoing effort to characterize mass accretion in young binary systems, we test the predictions of the binary accretion stream theory through continuous, multi-orbit, multi-color optical and near-infrared time-series photometry. Our observations from LCOGT, SMARTS, WIYN 0.9m, and ARCSAT provide detailed mass accretion rates and a measurement of the warm circumstellar material, all as a function of orbital phase. The predicted phase and magnitude of enhanced accretion are highly dependent on the binary orbital parameters and as such, our campaign focuses on 9 pre-MS binaries of varying periods and eccentricities. We present results highlighting the detection of consistent enhanced accretion events near periastron passages in eccentric binaries and strong sensitivity to the binary orbital parameters.
15:14 - 15:28 | Constraining the early stages of binary star evolution
Ed Gillen (University of Cambridge)
Abstract: There are only a dozen known low-mass, pre-main sequence eclipsing binaries (PMS EBs) with well-determined masses and radii. Detecting and characterising a sample of low-mass EBs sharing the same age and composition, yet spanning a wide range of masses, was one of the key motivations for CoRoT to observe the 3 Myr old NGC 2264 star forming region for 23 days in 2008. We identified 37 EBs among the possible cluster members and have performed an intensive program of ground-based follow-up observations to confirm their membership and determine their parameters. Furthermore, CoRoT observed the cluster again for 40 days in Dec 2011 - Jan 2012, as part of a coordinated campaign with Spitzer, Chandra, and a number of ground-based facilities including VLT/FLAMES, providing a unique simultaneous multi-band photometric and spectroscopic dataset.
I will present the sample of confirmed and candidate cluster members, detailing our innovative methods to determine their fundamental parameters using Gaussian process regression, before comparing our results to different models of stellar evolution. Of particular note, this sample includes: the first low-mass PMS EB with evidence of a circumbinary disk; two of the most extreme mass-ratio PMS EBs known; and a potential visual multiple containing three eclipsing signals. Taken together, this work offers an intriguing window onto the formation and early evolution of multiple star systems.
15:28 - 15:42 | Fundamental Calibrators for Stellar Evolution Models: New Eclipsing Binaries in Young Clusters Identified by K2
Trevor David (California Institute of Technology)
Abstract: Double-lined eclipsing binaries serve as fundamental calibrators for stellar evolution models. Benchmark grade calibrators (with mass and radius uncertainties of ~3%) having component masses below 1 solar mass are rare, particularly at pre-main sequence stages. We present the discovery and characterization of new eclipsing binaries in young stellar clusters, all identified by K2. In the 5-10 Myr old Upper Scorpius region, the nearest OB association, we present the lowest mass stellar eclipsing binary to date, with both components close to the hydrogen burning limit. Also in Upper Scorpius, we present evidence for a hierarchical triple with an eclipsing pair of brown dwarfs, only the second eclipsing brown dwarf pair known to date. In the 110-125 Myr old Pleiades cluster, only one eclipsing binary was known prior to the K2 mission. We present three new Pleiades eclipsing binaries, all with system masses less than 1 solar mass. We use these systems to critically assess stellar evolution models at low masses and young ages. K2 data in hand has also revealed new eclipsing and transiting systems in the moderately older (600-800 Myr) Hyades and Praesepe clusters.
15:42 - 16:16| Poster Pops & Coffee break
Poster Pop Presenters: TBD
16:16 - 16:30 | The role of dynamics in multiplicity
Richard Parker (Liverpool John Moores University)
Abstract: Binaries in the field are often used as a benchmark with which to compare binary populations in young star-forming regions. However, several studies have argued that dynamical interactions in star-forming regions can drastically alter the initial fraction, and orbital parameters of binary and multiple systems. In this talk I will show that basic information (i.e. position and membership probability) can be used to place constraints on the amount of dynamical evolution that a population of binaries has experienced in a given natal star-forming region. Based on observed properties of nearby star-forming regions, I will argue that the field binary population is likely to be indicative of the initial binary population as set by star formation. I will conclude by showing that the semimajor axis regime that observations are currently able to probe is, however, very sensitive to stochastic dynamical processes and that more observations are desperately required.
16:30 - 16:44 | Towards the True Binary Fraction of Very Low Mass Stars and Brown Dwarfs
Daniella Bardalez Gagliuffi (UC San Diego)
Abstract: Brown dwarf formation and how it differs from stars' is not well understood. Multiplicity is a key statistic for constraining theories on the formation and evolution of very low mass (VLM) stars and brown dwarfs. Most binary systems have been found through high resolution imaging, with a peak in the projected separation distribution at 1-4 AU coincident with the angular resolution limits of HST/ground-based adaptive optics. This suggests an observational bias and potentially significant underestimation of the VLM binary fraction, in conflict with formation theories. We have devised a separation-independent method to find potential binary systems using low-resolution, near-infrared spectra, which has uncovered over 60 VLM binary candidates to date. In order to find the true low mass star binary fraction, we are applying this method on an unbiased, volume-limited, spectroscopic sample of M7-L5 dwarfs, and following up candidates with high resolution imaging and spectroscopy to confirm their binary nature. Uncovering a large binary population would signify a different formation mechanism between stars and brown dwarfs.
16:44 - 16:58 | Properties of Stellar and Substellar Companions from the M-dwarfs in Multiples (MinMs) Survey
Kimberly Ward-Duong (Arizona State University)
Abstract: We present new findings from the M-dwarfs in Multiples (MinMs) Survey, a large-scale M-dwarf companion study of a complete volume-limited sample of 245 early M-dwarfs within 15 pc, as based on Hipparcos parallaxes. Through the addition of new high-resolution adaptive optics (AO) images to archival AO and digitized wide-field archival plate imaging, the entire sample is sensitive to companions at and beyond the bottom of the main sequence at projected separations of ~3 - 10,000 AU. The full sample is sensitive to 50 MJup companions beyond ~100 AU, and with 75% of the sample sensitive to 50 MJup between ~3 - 100 AU, we present a comprehensive survey searching for brown dwarf companions to M-dwarfs over the widest separation range to date. Our results include a new ~60 MJup companion at 7700 AU from its host star -- one of the widest M-dwarf+brown dwarf systems discovered -- and new close (< 50 AU) stellar and substellar candidate companions resolved with AO imaging. Multiple epochs of imaging with both high resolution and wide-field techniques for each target allows us to confirm or reject common proper motion candidates over the full separation range. Orbital fits are calculated for a subset of bound systems with multi-epoch astrometric measurements. By combining our measurements with reported detections from previous radial velocity searches, we present statistics over the broadest separation range for the companion frequency, separation distribution, mass ratio distribution, and frequency of hierarchical systems for M-dwarfs and M-dwarf+brown dwarf companions. These measurements extend our knowledge of companion properties for the lowest-mass stars, showing them to be distinct from their higher mass counterparts.
16:58 - 17:12 | Fundamental Stellar Parameters with HST/STIS Spectroscopy of M Dwarf Binaries
Sergio B. Dieterich (Carnegie Institution of Washington - Department of Terrestrial Magnetism)
Abstract: Mass is the most fundamental stellar parameter, and yet model independent dynamical masses can only be obtained for a small subset of closely separated binaries. The high angular resolution needed to characterize individual components of those systems means that little is known about the details of their atmospheric properties. We discuss the results of HST/STIS observations yielding spatially resolved intermediate resolution red optical stellar spectra for six closely separated M dwarf systems, all of which have precision dynamical masses for the individual components ranging from 0.4 to 0.076 MSol. We assume coevality and equal metallicity for the components of each system and use those constraints to perform stringent tests of the leading atmospheric and evolutionary model families throughout the M dwarf mass range. We find the latest models to be in reasonably good agreement with observations. We also discuss specific spectral diagnostic features such as the well-known gravity sensitive alkali lines and address ways to break the temperature-metallicity-gravity degeneracy that often hinders the interpretation of these features. This work is funded by NASA grant HST-GO-12938.
17:12 - 17:26 | Doubling Down: Individual Dynamical Masses for Ultracool Binaries
Trent Dupuy (University of Texas at Austin)
Abstract: Evolutionary models of stars at the bottom of the main sequence and into the substellar regime are widely used, but some of their basic predictions have remained essentially untested by direct measurement. The past several years have seen progress in testing models thanks to a growing number of dynamical (total) masses for brown dwarf binaries determined via (relative) orbit monitoring from ground-based adaptive optics (AO). However, the strongest tests of models require individual masses, particularly for calibrating the fundamental mass-luminosity relation. We present a new sample of individual dynamical masses for 17 binary systems, including objects down to ~30 M_Jup. These new results were only made possible by long-term orbit monitoring that combines resolved astrometry from Keck AO and HST with unresolved photocenter motion from our infrared astrometry program at the Canada-France-Hawaii Telescope over the past decade. We find that the mass-luminosity relation in the L/T transition is quite shallow, implying that the bulk substellar cooling decelerates as clouds disappear, perhaps even caused by this change at the surface. We also test whether models can reproduce the observed properties of each coeval, co-composition binary along a single isochrone. The ages derived from models using mass and luminosity are consistent with the age of the field population (>~1 Gyr) for most of our sample, as expected. Two of our sample binaries with spectral signatures of low surface gravity have model-derived ages of ~80 Myr and ~300 Myr, enabling the first tests of models using low-gravity ultracool dwarf dynamical masses.
17:26 - 17:30 | Closing Remarks
Abstract: Thanks for attending our session. Let's eat!